(1) Field of the Invention
The invention in general relates to dynamic range converters and more particularly to a digital dynamic range converter for digitally increasing or decreasing the dynamic range of an audio signal which is available in digital form. This converter is especially of the forward control type and is intended for use in digital audio equipment.
(2) Description of the Prior Art
Audio signals, such as, for example, music signals, have the property that their energy varies very strongly with time. Thus, such a signal comprises passages of very low energy and of very high energy. The passages of low energy are designated as soft passages and those of high energy are designated as loud passages. The energy of the audio signal is usually expressed in decibels (dB). The difference in energy between the loudest and the softest passages is designated as the dynamic range of the audio signal.
Let it be assumed that the softest passage of an audio signal has an energy A.sub.o and the loudest passage of such a signal has an energy B.sub.o. If this audio signal should be entirely audible, A.sub.o should at least exceed a given threshold value D.sub.L. The height of this threshold value is dependent upon the environment in which the acoustic version of the audio signal is listened to. This threshold value can be equal to the level of the environmental noise. In a living-room the environmental noise generally has a level of 30 to 40 dB and in a driven automobile approximately 80 to 95 dB.
In order to achieve that the energy of the softest passage just exceeds the threshold value used, the audio signal is supplied to an audio amplifier whose amplification factor can be varied by means of an adjustable volume control device. This amplifier delivers an amplified audio signal in which both the energy of the softest passage and the energy of the loudest passage exceed by an amount of D dB those in the original audio signal. This amount E depends upon the adjustment of the volume control device. The energy of the softest passage of the amplified audio signal is now equal to A.sub.o +E, whilst the energy of the loudest passage thereof is B.sub.o +E so that the dynamic range of this amplified audio signal is also equal to the dynamic range B.sub.o -A.sub.o of the original audio signal.
The requirement is often imposed on the amplified audio signal that the energy of the loudest passage must not exceed a given threshold value D.sub.H. Also this threshold value depends upon the environment in which the acoustic version of the audio signal is listened to. In a living-room, in which neighbours have to be considered, this threshold value will be approximately 70 dB, whilst in a driven automobile this threshold value can amount to approximately 110 to 120 dB. This means that in many cases the original audio signal need not only be amplified, but that also its dynamic range has to be increased to an amount D.sub.H -D.sub.L. Sometimes the dynamic range can be increased. A dynamic range converter is used to obtain this decrease or increase of the dynamic range.
Dynamic range converters are known in many forms. They can be roughly divided into dynamic range converters of the reverse control type and dynamic range converters of the forward control type. Independent of the type to which it belongs, in a dynamic range converter the original audio signal is multiplied by a control signal. It delivers an audio signal whose instantaneous value is constantly equal to the product of the instantaneous value of the original audio signal and the instantaneous value of the control signal. This control signal is derived from an auxiliary information signal. In the case of a dynamic range converter of the reverse control type, the absolute value of the output signal of the dynamic range converter is utilized as the auxiliary information signal. In a dynamic range converter of the forward control type, a rectified version of the original audio signal is used as the auxiliary information signal.
In recent years the interest in the digitalization of audio signals has strongly increased. A few years ago successful attempts had already been made to digitize an analogue music signal and to record it in this digital form on a magnetic tape and even on a disk-shaped carrier. In future it may be expected that even broadcasting stations will digitize their broadcast signals before transmitting them. Future audio equipment will thus increasingly be designed for receiving digital audio signals and for carrying out processing operations on these signals before these digital audio signals are converted in a digital-to-analogue converter into analogue audio signals which are suitable to be converted into acoustic signals.
In principle, any analogue information signal having a limited frequency spectrum can be digitized. For this purpose, it is sampled at regular instants. The number of samples taken per second is designated as the sampling rate and this number is at least equal to twice the highest frequency occurring in the frequency spectrum of this analogue information signal. Any signal sample thus obtained can be converted in an analogue-to-digital converter into a digital signal sample or code word having a fixed word length, that is to say having a fixed number of bits.
Notwithstanding the fact that the audio signal is now present in digital form, the aforementioned processing operations that will have to be carried out thereon will consist, for example, of a volume control, a bass-treble control, an equalization control and the aforementioned dynamic range conversion.
A dynamic range converter of the forward control type designed to digitally increase or decrease the dynamic range of an audio signal is known from the German Auslegeschrift No. 24 14 624.
In this known dynamic range converter in which the audio signal is available in digital form and is constituted by a sequence of audio signal samples, the following processing operations are carried out.
(i) The sequence of audio signal samples is converted into a sequence of unipolar signal samples which each represent the absolute value of an audio signal sample. PA0 (ii) The sequence of unipolar signal samples is converted into a sequence of control signal samples. PA0 (iii) Each audio signal sample is multiplied by an associated control signal sample. PA0 (a) it is provided with means for receiving adjustment quantities; PA0 (b) the processing operation for converting the sequence of unipolar signal samples into the sequence of control signal samples comprises the following steps:
The following processing operations are especially carried out for the conversion of the sequence of unipolar signal samples into the sequence of control signal samples.
The sequence of unipolar signal samples is subjected to a first-order recursive digital filtering and thus converted into a sequence of average value samples. This sequence is representative of the average value of the sequence of unipolar signal samples. PA1 Each average value sample is further subjected to an amplitude transformation and converted into a transformation sample. The relation between the magnitude of this transformation sample and the magnitude of the average value sample is described by a given fixed amplitude transmission characteristic curve. PA1 The sequence of transformation samples thus obtained is subjected to a first-order recursive digital filtering for producing the sequence of control signal samples which is thus representative of the average value of the sequence of transformation samples. PA1 (bi) a peak value detection step for converting the sequence of unipolar signal samples into a sequence of peak value samples; PA1 (bii) a non-linear amplitude transformation step for converting the sequence of peak value samples into a sequence of transformation samples, the relation between the magnitude of the transformation samples and that of the peak value samples being given by an amplitude transmission characteristic curve whose shape is determined by the adjustment quantities received.
This known dynamic range converter has a few essential disadvantages. In the first place, the sequence of average value samples responds far too slowly to abrupt and strong variations in the audio signal and in the second place it can be used only in those cases in which the dynamic range of the audio signal occurring at the output of the dynamic range converter must constantly be the same.